(a) Field of the Invention
The present invention relates to optical aligner using a compensation light and, more particularly, to an optical aligner suitably used for patterning a photoresist film on a semiconductor wafer.
(b) Description of the Related Art
In a photolithographic process used in fabricating a semiconductor device, a photoresist film is formed on an object film on a semiconductor wafer, followed by exposure of the photoresist film by using an optical aligner and development thereof to form a photoresist pattern on the photoresist film. The object film is then patterned using the photoresist pattern as a mask, to allow the object film to have a desired pattern. In the current photolithographic process, the reduction in the design rule of the semiconductor devices necessarily requires a higher-accuracy optical aligner.
FIG. 16 shows an outline of a conventional optical aligner in a sectional view thereof. The optical aligner 200 projects the image of a reticle 11 onto a wafer 21 mounted on a wafer stage 22 by using an exposure light. The wafer 21 includes a photoresist film made of a photoreceptor such as photoresist.
The optical aligner 200 includes an irradiation optical system 30 which irradiates an exposure light 52 irradiated by a light source 31 onto the reticle 11, and a projection optical system 40 which transmits the exposure light 52 passed by the reticle 11 onto the front face of the wafer 21 to project the image of the reticle 11 onto the wafer. The irradiation optical system 30 and the projection optical system 40 each include a variety of optical instruments such as optical lenses 32 to 34, 36 and a diaphragm 35.
In the conventional optical aligner 200, there is a problem in that a flare light occurs from the exposure light 52 due to diffraction of part of the exposure light in the reticle 11 and is incident onto the wafer 21 as a noise light. The flare light has a variety of light intensities on the surface of the wafer 21 depending on the ratio (opening ratio) of the transparent area to the total area of the circuit patterns on the reticle. For this reason, the resist pattern formed on the photoresist film of the wafer incurs a significant range of variation in the dimensions of the photoresist pattern after the development of the photoresist film, reflecting the variation of the light intensities of the flare light. The advance of the technique for achieving a smaller design rule in the semiconductor devices intensifies the influence by the flare light, which adversely affects on the dimensions of the photoresist pattern.
The flare light originates from a variety of factors, such as ununiform refractive index within the lens, unavoidable error in the shape of the lens surface, reflection on the wafer surface or lens surface, impurities adhered onto the lens surface. Thus, the measures have been taken heretofore to reduce the flare light as by improving the aberration, coating the lens surface and improving the machining accuracy for the lens surface.
Since the optical intensity of the flare light during the exposure is increased together with a smaller design rule, it is difficult to suppress the influence by the flare light while using the conventional techniques. For example, if an ArF excimer laser having an emission wavelength of 193 nm is used as the exposure light, the optical intensity of the flare light will be around ½ of the optical intensity of the exposure light on the wafer. There have been some reports on the fact where the variation of the dimensions in the photoresist pattern varies the line width in the semiconductor device to significantly degrade the product yield thereof.
Patent Publication JP-2004-62088A describes a technique for suppression of the flare light generated in the vicinity of a circuit pattern by forming a dummy pattern on the reticle in the vicinity of the circuit pattern.
The dummy pattern, if disposed in a space between adjacent circuit patterns, reduces the variation in the ratio of the transparent area to the total area including the transparent area and the opaque area in the reticle. It is noted that this technique is only effective in the case where a local flare light occurs having a smaller range of variation in the optical intensity, due to the restriction on the size or shape of the dummy pattern. That is, the flare light cannot be effectively suppressed in the entire pattern area of the reticle.